Recreactional vehicles such as three-and four-wheeled "all terrain vehicles" ("ATV's") commonly are used on a variety of harsh terrains including sand, gravel, grass, snow, mud, dirt, and even shallow water. The vehicles generally are powered by internal combustion engines linked to the drive wheels by conventional gear trains.
Variable speed split sheave V-belt clutch transmissions are well known and frequently are used on snowmobiles. These transmissions are desirable for a variety of reasons including their "automatic" nature; i.e., the operator need not shift the transmission through a series of gears because it is automatically continuously variable, and may automatically disengage the drive train by releasing the V-belt during idling (i.e., below a pre-determined RPM).
These V-belt transmissions would be desirable for use on all-terrain vehicles for the above-stated reasons. Because ATV's in fact are used on all types of terrain, the V-belt transmission would have to be protected from the obvious harm that mud, sand, and other debris would cause. Also, the V-belt should be kept dry to prevent slippage. Thus, the transmission should be entirely enclosed to keep it dry and clean. If the transmission is entirely enclosed, however, the heat generated by the V-belt cannot be effectively dissipated. Overheating may thus result, causing damage to either the V-belt or the transmission sheaves or both. By way of example, an enclosed transmission may heat up to about 200.degree.-250.degree. F. (90.degree.-120.degree. C.) while a typical V-belt will withstand only about 180.degree. F. (about 80.degree. C.). Overheating is most frequently encountered during high speed operation, but also is a problem under low speed/high load conditions such as are encountered in muddy or hilly terrains.